Plasma etching is replacing standard wet chemical etching methods in integrated circuit and thin film device processing because of the sharper edge definition and lower cost obtainable. Plasma etching is typically used to delineate fine line patterns in thin films of insulators such as SiO.sub.2 or Si.sub.3 N.sub.4 or metals such as Al or Cr on substrates such as Si or glass by removal of portions of the coating in a plasma discharge. The coated substrate is placed in a plasma chamber into which an inert gas is flowed at reduced pressure. In the presence of a plasma discharge, reactive species are generated from the inert gas by processes such as dissociative or impact ionization. The removal of the coating occurs by chemical reaction between the reactive species and the coating as well as by bombardment of the coating by ions present in the plasma. The gaseous reaction products formed by chemical reactions and ion bombardment of the coating are continuously removed from the chamber using a vacuum pump.
It is important that etching be terminated as soon as the desired pattern is properly defined since overetching causes undercutting of the etch mask; thus altering the dimensions of the desired pattern as well as attacking the substrate itself. Methods to control the etching process include monitoring the concentration of a reactive species or a reaction product in the plasma or monitoring the reflectivity of the surface of a substrate, such as a silicon wafer, to determine the end point, i.e., the point at which the coating material is removed, exposing the substrate.
Degenkolb et al, in Applied Spectroscopy 31, 40 (1977), have disclosed a plasma chamber with a window through which light emitted by different chemical species in the plasma passes. A wavelength selective light detector is positioned outside the window to measure the time dependent relative concentration of a particular species. In this approach, the concentration of a reactive species, as measured by the intensity of its optical emission, will decrease as it is used up in the etching process and then return to about its initial concentration when the etching process is complete. The intensity of the emission from a reaction product, on the other hand, will increase as more of it is produced during etching and then decrease when the etching process is complete.
Busta et al, in Industrial Research and Development, June 1978 at page 133, have also disclosed a plasma chamber with a window. A light source and detector positioned outside the window are used to monitor changes in the reflectivity of the surface of a substrate situated within the chamber. A change from one reflectivity value to another will indicate that a coating layer has been removed from the substrate.